Image intensifier including polyimide support

ABSTRACT

A screen which comprises a support on which is arranged a layer of a substance capable of fluorescing has the support formed of a polyimide. The screen is suitable for use in a vacuum image intensifier.

United States Patent Inventors Karl Franz;

Gerhard Kochmann, both of Erlangen,

Germany Appl. No. 812,543

Filed Mar. 11, 1969 Patented Nov. 2, 1971 Assignee SiemensAktiengesellschaft Erlangen, Germany Priority Mar. 14, 1968 GermanyIMAGE INTENSIFIER INCLUDING POLYIMIDE SUPPORT [56] References CitedUNITED STATES PATENTS 2,945,976 7/1960 Fridrich et al. 313/108 A3,254,252 5/1966 Anderson et al.... 313/94 3,403,279 9/1968 Wanmaker etal 313/94 OTHER REFERENCES Deshotels et al; Chemical Abstracts, Vol. 65,1966; 14581 d Primary Examiner-Robert Sega] Attorney- Richards & GeierABSTRACT: A screen which comprises a support on which is arranged alayer of a substance capable of fluorescing has the support formed of apolyimide. The screen is suitable for use in a vacuum image intensifier.

PATENT Enuov 2 m: 3,6 17, 791

INVENTORS:

K. F/anz 4 GKOc/Ima/m I ATTOILNEBS IMAGE INTENSIFIER INCLUDING POLYIMIDESUPPORT This invention relates to a screen capable of fluorescing, e.g.,a screen capable of making visible invisible images, in which the screencomprises a layer of fluorescent substance arranged on a support.

Fluorescent screens of this type are used in vacuum image intensifiers,in which a photocathode is situated in an evacuated flask or bulb behinda ray-entry surface, which may be employed for the conversion ofinvisible ray images, for example X-ray images, into electron images.Behind the cathode are electron-optically active electrodes and afluorescent screen on which visible images are formed by the electronsemitted by the photocathode and focused by the voltages applied to theelectrodes.

Known fluorescent screens by which are meant screens which fluorescewhen they absorb radiation, which are used in, for example, imageintensifiers, usually have glass, mica or aluminum oxide foils which actas a support on which the fluorescing layer is applied. However, micasuffers from the disadvantage that it gives off water ofcrystallization, for example when being used under high vacuum atelevated temperatures and when being bombarded with electrons, and thusit can cause a decrease in transmission as well as a deterioration ofthe vacuum. The mica does, in fact, become cloudy because of the waterof crystallization being given off. Furthermore, mica has only a smalldegree of hardness and is thus a material which can easily be damaged,together with the fluorescent coating. in addition, because of itslaminated structure, flaking can easily occur, especially at the edges.

it is also known that aluminum oxide foils can only be obtained freefrom holes and with an optically satisfactory quality with very greatdifficulty. In addition, these foils have a low mechanical strengthwhich presents difficulties during the handling thereof in the workingoperations.

Consequently, foils consisting of glass are used as the transparentsupports in most cases. However, these foils suffer from thedisadvantage that they produce a reduction in contrast when they havethe necessary thickness to give sufficient strength for handlingpurposes. This is because the luminescent light of the fluorescent layeris reflected at the boundary surfaces of the support in such a mannerthat the light is spread out disturbingly in the support parallel to theimage surface. The resultant brightening of the background causes adeterioration in the clear image formation, because of the reduction inthe contrast on account of the brightening effect. Since this impairmentof the image formation depends mainly on the thickness of the support,efforts have been made to use glass foils which are as thin as possible.However, the reduction in layer thickness is limited, because of thenecessary mechanical strength and the methods by which these foils maybe produced, to about 100p.

it is therefore an object of the present invention to provide afluorescent screen in which the aforementioned disadvantages aredeviated or reduced.

According to one aspect of the present invention, there is provided ascreen which comprises a support on which is disposed a layer ofsubstance capable of fluorescing, characterized in that the support isformed of a polyimide.

Foils formed of polyimide have proved very satisfactory under highvacuum as supports for fluorescent layers, because they are stable up totemperatures of 350 C. under vacuum, are not adversely affected by theloading caused by ionizing radiation, e.g., when used in X-ray imageintensifiers, and are resistant to those chemicals which are generallyemployed in coating with fluorescent substance. Furthermore, polyimidefoils still have sufficient strength, even when their thickness is below15 1.. Accordingly, they provide the possibility of improving thecontrast which can be obtained. Polyimide foils are, in addition,deformable and can be produced easily in different shapes, whereby theshapes normally employed for supports of fluorescent screens may beeasily obtained, for example, hemispheres which are usually employedwith vacuum image intensifiers.

Fluorescent screens having a polyimide foil as the support may be unitedwith the glass flasks used in image intensifiers, as well as with othermaterials, in a manner resisting high vacuum and temperature by the useof various adhesives, e.g., polyimide lacquers and polysiloxanes. Inthis manner, it is possible to obtain good optical contact whilesubstantially avoiding reflecting surfaces. As a result of the avoidanceof reflections, an additional improvement in the image is obtained. Itis an additional advantage of the polyimide foils that they aregas-permeable. This property is advantageous when the free surface ofthe fluorescent layer is to be covered in a known manner by flrstlycovering the surface of the fluorescent layer with a smoothingintermediate lacquer layer and thereafter vapor-coating with aluminum.The intermediate lacquer layer which is covered with aluminum is thenremoved by heating. When the polyimide foil is used, the decompositionproducts of the lacquer layer can diffuse off through the foil. Thisavoids both a disturbance of the structure of the fluorescent layer, andthe formation of bubbles on the aluminum skin. Polyimide foils aresubstantially impermeable to shortwave ultraviolet radiation and,consequently, when they are used with fluorescent layers which containzinc sulphide, any damage to the layer of fluorescent substance by theshortwave rays is avoided. Since the polyimide foils can be obtained orproduced in the form of synthetic plastics sheets having a large area,fluorescent screens of an equally large area can be made, out of whichfluorescent screens of the desired size may then be cut.

An example of a polyimide foil which is suitable for use in the screenaccording to the invention may be a material obtained for example, bythe polycondensation of a tetrabasic aromatic acid with an aromaticdiamine. Because of the favorable combination of mechanical andelectrical properties, there are especially suitable for the use assupporting foils for example are produced by polycondensation of atleast one of the group including 2,3,6,7-naphthalene tetracarbonic aciddianhydride, 3,3',4,4'-diphenyltetracarbonic acid dianhydride,2,6-dichloronaphthalene-1,4,5,8-tetracarbonic acid dianhydride,pyrrolidin-2,3,4,5-tetracarbonic acid dianhydride with at least one ofthe group including m-phenylenediamine, p-phenylenediamine,4,4'-diamin0diphenylpropane, 4,4-diaminodiphenylmethane,4,4'-diaminodiphenylether, l,5-diaminonaphthalene,3,3-dichlorobenzidine.

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawing which is a cross-sectional viewthrough a vacuum image intensifier.

In the vacuum image intensifier generally indicated by the referencenumeral 1, a photocathode 3 is disposed behind a ray-entry window 4 inan evacuated glass flask 2. Behind the cathode 3 are electrodes 5 and 6,which consist of sheet metal strips bent in the form of a ring. Afluorescent screen 8 is situated adjacent an end window 7 of the flask2, which is opposite the entry window 4. An anode 9 is disposed in frontof the fluorescent screen 8 and faces the interior of the flask. Theanode 9, photocathode 3 and electrodes 5 and 6 constitute theelectron-optical system of the image intensifier.

The photocathode 3 consists of a polyimide support 10 which has athickness of less than 10p. and which is covered on that surface facingthe window 4 with a fluorescent layer 11 which contains a polysiloxanebinder in addition to cadmium sulfide which acts as the fluorescentsubstance. The free surface of the fluorescent layer 11 is covered witha reflecting layer 12 consisting of a white pigment, in this casemagnesium oxide. On that side of the support 10 remote from the entrywindow 4 is a photocathode layer 13, consisting of antimony activatedwith caesium. The photocathode 3 is fixed on the walls of the flask 2 bymeans of holder devices, two of which 14 and 15 are shown on two sidesof the photocathode 3.

The fluorescent screen 8 comprises a support 16, a fluorescent layer 17which contains silver-activated zinc-cadmium sulfide and silicon dioxide(as a-binder), and an aluminum layer 18. The complete screen 8 isarranged so that the free surface of the support 16, with the aid of anadhesive layer 19 consisting of polyimide lacquer, is in good opticalcontact on the inside of the end wall 7 of the flask 2.

Use of the intensifier enables an X-ray image to be made visible. TheX-rays pass through the window 4 into the flask 2 where they penetratethe reflecting layer 12 and generate mainly light in the fluorescentlayer 11. This light passes through the transparent support and producesan electron image in the photocathode layer 13, corresponding to thelight distribution. Suitable voltages applied in a known manner to theelectrodes 5, 6 and the anode 9, as well as to the cathode 3, cause anelectron image to be formed on the screen 8, where they produce in thelayer 17 the intensified fluorescing and visible image.

We claim:

1. A vacuum image intensifier which comprises an evacuated flask inwhich are contained (a) a photocathode disposed behind a ray-entrywindow of the evacuated flask and including a support on that side ofwhich facing the ray-entry window is disposed a layer of a substancecapable of fluorescing and on that side of which remote from theray-entry window is disposed a layer of a substance capable ofphotoemission; (b) an electrode capable, in use, of acceleratingelectrons emitted at the photocathode; (c) an electrode capable, in use,of focusing said electrons: and (d) an electron-reactive screen whichcomprises a support on which is disposed a layer of a substance capableof fluorescing, characterized in that the support of saidelectron-reactive screen is formed of a polyimide, said polyimide beingformed by the condensation of a tetrabasic aromatic acid which is atleast one of the group including 2,3,6,7-naphthalene tetracarbonic aciddianhydride 3,3',4,4'-diphenyltetracarbonic acid dianhydride2,6-dichloronaphthalene-l,4,5,8-tetracarbonic acid dianhydride andpyrrolidin-2,3,4,5-tetracarbonic acid dianhydride;

and an aromatic diamine which is at least one of the group includingm-phenylcnediamine, p-phenylenediamine, 4,4-diaminodiphenylpropane,4,4'-diaminodiphenylmethane 4,4'-diaminonaphthalene,1,5-diaminonaphthanlene,

and

3,3'-dichlorobcnzidine.

2. A vacuum image intensifier according to claim 1, wherein the supportof the photocathode is formed of a polyimide.

3. A vacuum image intensifier according to claim 1, wherein theelectron-reactive screen is adhered to that wall of the evacuated flaskremote from the ray-entry window, by a polyimide lacquer layer.

2. A vacuum image intensifier according to claim 1, wherein the supportof the photocathode is formed of a polyimide.
 3. A vacuum imageinteNsifier according to claim 1, wherein the electron-reactive screenis adhered to that wall of the evacuated flask remote from the ray-entrywindow, by a polyimide lacquer layer.